The present invention relates to an air-fuel ratio control system for an internal combustion engine, which system controls the air-fuel mixture to the stoichiometric air-fuel ratio at which ratio a three-way catalyst acts most effectively.
In a known air-fuel ratio control system for a motor vehicle, the air-fuel ratio of the air-fuel mixture burned in the engine cylinders is detected as the oxygen concentration in the exhaust gases by means of an O.sub.2 sensor provided in the exhaust system of the engine, and a decision is made dependent on the output signal from the O.sub.2 sensor which indicates whether the air-fuel ratio is richer or leaner than the value corresponding to the stoichiometric air-fuel ratio, for producing a control signal. The control signal is applied to a proportion and integration circuit (PI circuit), the output of which is applied to a comparator. The comparator compares the output of the PI circuit with a triangular pulse train to produce square wave pulses. The pulses operate an electromagnetic valve so as to control the amount of bleed air in a carburetor for controlling the air-fuel ratio of the mixture.
FIG. 3 shows waveforms at the comparator. Reference PI designates an output of the PI circuit (hereinafter called PI value) and T shows the triangular pulse train. The comparator produces square pulses SP as a result of the comparison. As seen from the figure, the duty ratio of the square pulses is determined by the level of the PI value. The inclination of the PI value increases with the increase of the constant of the PI circuit. Accordingly, if the constant is increased, the duty ratio quickly changes. When the duty ratio of the pulses is reduced, the air-fuel mixture is enriched. Thus, the air-fuel ratio can be controlled to the stoichiometric air-fuel ratio at which a three-way catalyst in the exhaust system acts most effectively. In such an air-fuel ratio control system, when the vehicle is accelerated, the air-fuel ratio is liable to deviate from the stoichiometric air-fuel ratio.
In order to rapidly converge the deviated air-fuel ratio to the stoichiometric air-fuel ratio, the constant of the PI circuit is changed to a large value. The constant of the PI circuit is stepwisely changed to several values in accordance with driving conditions of the vehicle. The constant of the PI circuit is decreased to a small value at engine idling operation, because the air-fuel ratio does not vary much at idling.
Accordingly, there is provided a first constant for idling operation, second constant for steady state at which the vehicle is driven at a constant speed, and third constant for acceleration of the engine. The second constant is selected to have a value between the first and third constants.
On the other hand, generally, a carburetor does not have a flat load characteristic. Namely, when the engine is accelerated, the supply of fuel delays, rendering the air-fuel mixture lean. Accordingly, if the engine is accelerated during operation within an acceleration range controlled by the second constant, the air-fuel ratio does not quickly respond to the acceleration because of the small constant. However, if the second constant is set to a larger value, the air-fuel ratio changes a lot in response to a small acceleration. Such an operation causes overshooting of the feedback control, which renders the driveability of the vehicle and emission control poor.